1. Field of the Invention
The present invention relates to a wire electric discharge machine, and more specifically to a wire electric discharge machine having a taper machining function for machining a workpiece between a first plane and a second plane such that a machining shape on the first plane and a machining shape on the second plane are different.
2. Description of Related Art
Taper machining with a wire electric discharge machine is known. Normally, machining with a wire electric discharge machine is carried out with a workpiece set on an upward-facing workpiece setting surface. As a result of taper machining, the workpiece has non-congruent shapes at an upper surface and at a lower surface. FIG. 1 schematically shows such taper machining with cross-sectional planes parallel to the direction of an arrow. As understood from FIG. 1, regarding a movement in each block of a machining program, generally, the movement distance (machining distance) Lu at the upper surface of the workpiece and the movement distance (machining distance) L1 at the lower surface are different. This means that the relative speed between the wire electrode and the workpiece, or in other words, the machining speed is different on the workpiece upper surface and on the workpiece lower surface. In electric discharge machining, when the relative speed between the wire electrode and the workpiece is lower, the number of times that the electric discharge is produced is larger, so that a greater electric discharge gap is produced.
In this situation, control on the relative speed between the wire electrode and the workpiece is performed by setting an intermediate plane located midway between the upper and lower surfaces of the workpiece and parallel to the upper and lower surfaces, and controlling the relative speed between the wire electrode and the workpiece at this intermediate plane by commands fed according to the program. In this specification, this intermediate plane will be called “speed control plane”. In taper machining, the machining distance Lm on the speed control plane is greater than the machining distance Lu at the upper surface and smaller than the machining distance L1 at the lower surface. Also the relative speed between the wire electrode and the workpiece on the speed control plane is a value intermediate between the value at the upper surface and the value at the lower surface.
The electric discharge gap itself exists no matter whether it is taper machining or not. Normally, the electric discharge gap is estimated as an amount to be added to a difference between the position of the wire and the position of the machining plane coming from the fact that the wire has a finite diameter. Generally, the correction to deal with the deviation comprised of both is called “wire-diameter correction”. In other words, the “wire-diameter correction” intends to rectify a deviation of the “actual machining path” from the “machining path designated by the machining program”, estimated as “wire diameter+electric discharge gap”.
In the prior art, wire-diameter correction in taper machining was performed by setting a wire-diameter correction amount suited for the speed control plane and using it as a wire-diameter correction amount common to the workpiece upper and lower surfaces. However, for the reason described above, the actual electric discharge gap in taper machining is different on the workpiece upper surface and on the workpiece lower surface. Thus, as shown in FIG. 2 which is a cross-sectional view perpendicular to the direction of the arrow in FIG. 1, at the face where the machining speed is relatively low (in this instance, the upper surface), excessive machining (producing an over-cut) occurs, while at the face where the machining speed is relatively high (in this instance, the lower surface), deficient machining (leaving an uncut part) occurs. There is found no known publication describing a technique for easily solving such problem in taper machining.